1. Field:
This invention relates to the recovery of monomeric components from condensation-type linear polyester resins such as high molecular weight polyethylene terephthalate (PET), and the repolymerization of such monomers.
2. State of the Art:
Prior efforts regarding reuse of high molecular weight polymer PET have been directed to reuse either in the existing polymerized form, e.g. by grinding and using as filler material, etc. or by partially depolymerizing in the presence of a monomer to a limited extent to produce a low molecular weight polymer, e.g. as disclosed in U.S. Pat. No. 3,703,488 of Morton.
Morton suggests that complete depolymerization to a terephthalic acid or dimethyl terephthalate has been proposed but has not been satisfactory. Morton refers to British Pat. No. 610,136 as disclosing a depolymerization process using large excesses of glycol. The process of Morton, however, depolymerizes scrap PET merely to a lower molecular weight PET resin by high temperature extrusion of the solid, scrap PET resin with significant addition of glycol. Such extruded low molecular weight PET resin is included in amounts up to about 20% by weight with fresh glycol esters or fresh prepolymers so that objectionable color in the final product can be avoided.
Depolymerization of PET is suggested in MacDowell, U.S. Pat. No. 3,222,299. MacDowell describes a process for heating scrap PET in the presence of glycol to form a diglycol terephthalate, which can be used in minor amounts in a polyesterification process for forming PET frcm freshly prepared diglycol ester of terephthalic acid in the presence of an aromatic sulfonate compound containing an alkali or alkaline earth metal.
In MacDowell's process, all the catalysts, dyes, pigments, stabilizers, etc. present in the fibrous waste polyester are fed into the prepolymerizer and into the final polymerizer. Furthermore, the diglycol therephthalate ester prepared in the reactor of MacDowell requires quantities of fresh dimethyl terephthalate to be introduced into the ester exchange column. Also significant quantities of fresh glycol, actually equimolar amounts with respect to the fresh dimethyl terephthalate, must be introduced into the reactor in order to form diglycol terephthalate. Therefore, the catalysts, dyes, pigments, stabilizers, etc. present in the original solid scrap PET are present in the final polymer except their concentrations in the final polymer are only one-half their concentration in the original scrap PET.
MacDowell further discloses that polyesterification of the diglycol terephthalate does not occur unless sodium 3,5-dicarboxybenzenesulfonate is present.
U.S. Pat. No. 3,988,406 of Nakamura, et al. also utilizes a process somewhat similar to MacDowell for causing some depolymerization of scrap PET in the presence of substantial quantities of ethylene glycol after the scrap PET fibers have been formed into a fibrous rod about six millimeters in diameter and about 30 millimeters in length.
Numerous patents disclose various treatments of PET scrap to utilize such scrap in an economic manner. Patents disclosing such techniques are as follows: U.S. Pat. No. 4,193,896 of Cook; U.S. Pat. No. 4,368,274 of Scott; and U.S. Pat. Nos. 4,003,880; 4,003,881; and 4,118,187 of Sidebotham, et al.